Flexible electronic products are required to be light, portable and flexible so that they can be warped and bended without mal-functionality. Owing to the ionic bond structure in the oxide semiconductor TFT, the on-current characteristics of the TFT are less sensitive to the warping of the substrate. Moreover, since the oxide semiconductor TFT exhibits higher carrier mobility (about 10 cm2/V-sec) than the amorphous (a-Si:H) TFT and can be manufactured under a low temperature, it is very suitable for use in flexible electronic applications.
To enhance the current density of the TFT, U.S. Patent Appl. No. 20050199879 discloses a TFT using a dual-gate structure on the top and bottom surfaces of the active layer so as to enhance the current density by building up more carrier transport paths, as shown in FIG. 1A and FIG. 1B. In FIG. 1A, the TFT 100 comprises a substrate 102, two gate electrodes 103, 104, two gate dielectric layer 105, 106, an active layer 107, a source electrode 108 and a drain electrode 109. During the operation of the TFT 100, a channel is induced on each of the surfaces of the active layer by applying a bias voltage across the gate electrode 103/104 and the active layer 107 to enhance the current density. In FIG. 1B, the TFT 101 is similar to the TFT 100 in FIG. 1A except that the order for forming the drain/source 108/109 and the active layer 107 is different. The TFT 100 and the TFT 101 in FIG. 1A and FIG. 1B are problematic because they are manufactured by lots of steps, which cause complicated processing as well as increased cost.
Therefore, the present invention provides an oxide semiconductor TFT comprising a composite semiconductor active layer to enhance the current characteristics of the oxide TFT, enhance stable driving current and reduce unstable current due to oxygen vacancies. Moreover, by adjusting the thickness of the semiconductor active layer, the leakage current is kept low.